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Polymer Testing 28 (2009) 215–222

Contents lists ava

Polymer Testing

journal homepage: www.elsevier .com/locate/polytest

Material Behaviour

Crystallization kinetics of polypropylene/ethylene–octenecopolymer blends

Petr Svoboda a,*, Dagmar Svobodova a, Petr Slobodian a, Toshiaki Ougizawa b,Takashi Inoue c

a Faculty of Technology, Tomas Bata University in Zlin, nam. TGM 275, 762 72 Zlin, Czech Republicb Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1-S8-33 Ookayama, Meguro-ku, Tokyo 152-8552, Japanc Department of Polymer Science & Engineering, Yamagata University, Yonezawa 992-8510, Japan

a r t i c l e i n f o

Article history:Received 3 November 2008Accepted 16 December 2008

Keywords:PolypropyleneEthylene–octene copolymerCrystallizationPhase morphology

* Corresponding author. Tel.: þ420 576 031 335; faE-mail address: svoboda@ft.utb.cz (P. Svoboda).

0142-9418/$ – see front matter � 2008 Elsevier Ltddoi:10.1016/j.polymertesting.2008.12.007

a b s t r a c t

Blends of polypropylene and ethylene–octene copolymers (EOC) were investigated bytransmission electron microscopy, optical microscopy and differential scanning calorim-etry (DSC). The main focus was on phase morphology and crystallization for blends con-taining EOC with different octene content (28, 37 and 52 wt.%). Also, for a given octenecontent (37 wt.%), the effect of molecular weight (115, 180, 229k) of EOC on morphologywas observed. The largest particles were found in the blend with EOC-28 and the smallestwith EOC-52. This blend with the smallest particles exhibits the fastest crystallizationkinetics by two independent methods, optical microscopy and DSC. This behavior wasexplained by a model. Crystallizing polypropylene lamellae have to travel a longer distancegoing around large particles, which slows down overall crystallization growth rate. In thecase of smaller particles, the obstacles are smaller and the crystallization is faster.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Polypropylene (PP) is one of the most versatilecommodity polymers because it possesses exceptionalproperties, including excellent chemical and moistureresistance, good ductility and stiffness, and low density. It isalso easy to process and relatively inexpensive. It is wellknown that good properties of PP as an engineering poly-mer are seriously limited by its low impact resistance. Toimprove the impact resistance of the PP matrix, rubbershave been used as impact modifiers. Extensive research hasbeen published on the blends of PP with ethylene–propylene rubber (EPR), ethylene–propylene diene copol-ymer (EPDM) and styrene–ethylene–butylene–styrene

x: þ420 577 210 172.

. All rights reserved.

copolymer (SEBS). Recently, interest has centered on theuse of ethylene–octene copolymer (EOC) [1–29].

Many researchers have observed the improvement inimpact strength of PP with the addition of EOC [3,5–8,12,17,22,25]. The size of spherulites also affects the impactstrength and, therefore, some researchers have investi-gated crystallization of these PP/EOC blends in detail[4,16,19,23,26,28], with some having used a nucleationagent to decrease the spherulite size and thus increase theimpact strength [6,26]. Other researchers have investigatedhow the addition of various EOCs would affect the rheologyof these blends [7,14–17,20].

Dow Chemicals is nowadays producing quite a largenumber of EOCs with the trade name ENGAGE� coveringa range of density and melt flow index. Carriere et al. [21]have shown that increasing octene content in EOC haslowered the interfacial tension in PP-EOC systems. Howwould that affect the actual phase morphology and crys-tallization? To our best knowledge, such systematic

Table 1List of used ethylene/octene copolymers.

Name Octenea Mna Ethylene Octene et/oct ratio

wt.% g mol�1 mol.% mol.%

EOC-28-131 28 131000 91.14 8.86 10.3EOC-37-115 37 115000 87.20 12.80 6.8EOC-37-180 37 180000 87.20 12.80 6.8EOC-37-229 37 229000 87.20 12.80 6.8EOC-52-130 52 130000 78.69 21.31 3.7

a Data were supplied by the manufacturer.

P. Svoboda et al. / Polymer Testing 28 (2009) 215–222216

detailed transmission electron microscopy (TEM) studycombined with optical microscopy and differential scan-ning calorimetry (DSC) measurement has not yet beendone and, therefore, was investigated in this research work.

The softness, high temperature resistance and impactstrength should be closely related to the morphology. In thispaper, we deal with the basic aspect of the morphologygeneration. We have chosen five EOC grades to be tested withPP, more particularly three different octene contents –28, 37and 52 wt.% –to see the effect of octene content, and thenthree different molecular weights Mn (115, 180 and 229k)with constant octene content (37 wt.%) to see the effect of Mn

on morphology and crystallization kinetics measured byoptical microscopy and by DSC.

Fig. 1. TEM micrographs of PP/EOC 50/50 blends after mixing at 100 rpm for7 min at 200 �C and quenching to 0 �C water. The octene content in EOC was(a) 28 wt.%, (b) 52 wt.%.

Fig. 2. TEM micrographs of PP/EOC 50/50 blends after mixing at 100 rpm for7 min at 200 �C and quenching to 0 �C water. The octene content in EOC wasconstant 37 wt.%. Molecular weight Mn of EOC was (a) 115k, (b) 180k, (c)229k g mol�1.

2. Experimental

The isotactic polypropylene (PP) was a commercialpolymer supplied by Mitsui Chemicals Inc. (J3HG,Mw ¼ 3.5 � 105 g mol�1 and Mn ¼ 5 � 104 g mol�1).

Ethylene–octene copolymers were special samplesprepared by Dow Chemicals. Table 1 shows the octenecontent in weight and molar %, ethylene/octene molar ratioand molecular weight Mn.

The PP and EOC were melt-mixed (charge 0.7 g) at 200 �Cfor 7 min at 100 rpm in a miniature mixer, Mini-Max Moulder(model CS-183MMX, Custom Scientific Instruments, Inc.).

Fig. 3. Growth of spherulite in time at Tc ¼ 135 �C in 50/50 blend of PP/EOC-37-229 by polarized optical microscopy after 1 min pre-heating at 200 �C.

Time/min

0 2 4 6 8 10 12 14

R / µm

0

5

10

15

20

25

30

EOC-28-131EOC-52-130EOC-37-180

2.030

2.748

Spherulite growth rateG=dR/dt (µm/min)

1.810

Fig. 4. Three examples of spherulite growth rate evaluation from the plotradius of spherulite vs. time for PP/EOC 50/50 blends at Tc ¼ 135 �C.

P. Svoboda et al. / Polymer Testing 28 (2009) 215–222 217

Only one blend ratio was used – 50/50 wt.%. The melt-mixedblend was extruded and the extruded string was quenched inice-water (0 �C).

For the transmission electron microscopy (TEM) anal-ysis, the specimens were microtomed to an ultrathinsection of about 70 nm thick using a Reichert–Jung ultra-cryomicrotome with a diamond knife at �80 �C. Thesections were then stained with RuO4 vapor at roomtemperature for 2 h. The structure was observed by anelectron microscope, JEM 100CX (100 kV).

In a differential scanning calorimeter (Seiko Instru-ments –EXSTAR 6000), the specimens were heated ina nitrogen atmosphere. For linear temperature increase ordecrease the rate was 10 �C min�1.

For the optical microscopy observations, the specimenwas melt-pressed between two cover glasses on a hot stage

Temperature / °C

80 85 90 95 100 105 110 115 120 125 130

Exo

th

erm

/ µW

5000

10000

15000

20000

25000

30000

35000EOC/PP (50/50)

EOC-52-130

EOC-37-115

EOC-37-180

EOC-37-229

Fig. 5. Crystallization of PP/EOC 50/50 blends by DSC during temperaturedecrease 10 �C/min after 1 min pre-heating at 200 �C.

Table 2Crystallization kinetics G from optical microscopy and crystallizationtemperatures Tc from DSC.

Name G Tc

mm min�1 �C

EOC-28-131 2.039 105.20EOC-37-115 2.078 106.06EOC-37-180 1.884 105.04EOC-37-229 2.191 106.43EOC-52-130 2.763 110.55

1.8

2.0

2.2

2.4

2.6

G / µm

m

in

-1

a

P. Svoboda et al. / Polymer Testing 28 (2009) 215–222218

at 200 �C. The melted specimen was then placed ontoa LINKAM hot stage of the microscope set to 135 �C. Struc-tural development during the isothermal annealing wasobserved under both the optical and the polarizing micro-scope (Olympus BH-2) equipped with a video recordingsystem and exposure control unit (Olympus PM-20).

106

107

°C

37 wt% of octeneb

3. Results and discussion

In Fig. 1, two TEM pictures of the PP/EOC (50/50) arecompared. The molecular weight of EOC was almost

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

G / µm

m

in

-1

130k

115k131k

wt% of octene

25 30 35 40 45 50 55

Tc/ °C

105

106

107

108

109

110

111

112

130k

131k115k

a

b

Fig. 6. Crystallization kinetics from optical microscopy (a) and from DSC (b)as a function of octene content in EOC for PP/EOC 50/50 blends. The struc-ture of the three corresponding blends is shown by 3 inserted TEM pictures.

100 120 140 160 180 200 220 240

105

Tc /

Mn x 10

-3 / g mol

-1

Fig. 7. Crystallization kinetics from optical microscopy (a) and from DSC (b)as a function of molecular weight Mn of EOC for PP/EOC 50/50 blends.

precisely the same for both of them. The mixing conditions(time, temperature, rotor speed) were also the same forboth of them. Then, one is able to evaluate the influence ofoctene content on resulting morphology. The blend con-taining EOC with 28 wt.% of octene has much larger parti-cles (Fig. 1a). The RuO4 preferably stains EOC, so the darkphase can be assigned to be the EOC. The blend in Fig. 1ahas ethylene–octene copolymer as the continuous phasewhile for the blend containing EOC with 52 wt.% of octeneit is just the opposite – the continuous phase is poly-propylene (bright phase), see Fig. 1b.

To observe the effect of molecular weight of EOC on finalmorphology, we have used the EOC with fixed octenecontent (37 wt.%) with three different molecular weights:115k, 180k and 229k g mol�1. The comparison is shownin Fig. 2. As expected, there is a huge effect on finalmorphology. Fig. 2a shows the blend containing EOCwith the lowest Mn – 115k. The viscosity of the EOC at themixing temperature was lower than the viscosity of thePP, which resulted in EOC being the continuous phase(or matrix).

Which phase is continuous has a huge influence on thefinal mechanical properties and, therefore, this knowledgeis essential in material design. It is generally accepted thatthe properties of a blend are governed mainly by the

Fig. 8. Growth of spherulites in time at Tc ¼ 135 �C in 50/50 blend of PP/EOC-28-131 (a) and EOC-37-180 (b) by polarized optical microscopy after 1 minpre-heating at 200 �C.

P. Svoboda et al. / Polymer Testing 28 (2009) 215–222 219

Fig. 9. Model for crystallization: (a) no obstacles, (b) small particles, (c) largeparticles.

P. Svoboda et al. / Polymer Testing 28 (2009) 215–222220

properties of the matrix. In our case, the PP is a hard ductilepolymer while EOC is a soft rubbery polymer. The elasticproperties should be greatly affected by the morphology –if the matrix is ductile PP or elastic EOC.

The morphology of the PP/EOC-37-180 blend (in Fig. 2b)is quite unique and was observed only for this particular Mn

and octene content. The structure looks like it has a layeredmorphology, something like stripes of individual polymerswith a thickness about 1–2 mm. Even though there are alsospherical particles, their content seems to be inferiorcompared to the stripes.

Coming to the EOC with the highest Mn (229k) in thisseries (Fig. 2c), the PP became the continuous phase withdispersed EOC particles. Having fixed octene content, one isable to change the morphology of PP/EOC blend (quitesignificantly) by changing the molecular weight of EOC asshown in Fig. 2a–c.

Optical microscopy equipped with a precise tempera-ture controlled hot stage is a powerful tool for observationof the crystallization of spherulites at a constant elevatedtemperature. Does the presence of EOC affect the spherulitegrowth rate of PP? Fig. 3 shows the example of spherulitegrowth in PP/EOC blend for isothermal crystallization at135 �C. Even though there are clearly two phases, as shownin Fig. 2c, the PP spherulite does grow through the phaseseparated structure.

One can measure the radius of the spherulite (or moreconveniently at first the diameter) and plot it as a functionof time, as shown in Fig. 4. Then, the slope of the line givesus the spherulite growth rate that can be compared for theinvestigated blends. About 5–7 spherulites were measuredfor each blend to get the average growth rate and a stan-dard deviation.

The crystallization kinetics was measured also byanother independent experiment – DSC with temperaturedecrease of 10 �C min�1, as shown in Fig. 5. The higher thecrystallization temperature Tc the faster was crystallization.For each blend, the DSC was measured five times to get theaverage Tc and standard deviation.

Table 2 and Figs. 6 and 7 summarize the results fromtwo independent experiments – optical microscopy at135 �C and DSC. First, Fig. 6 shows the effect of octene oncrystallization kinetics. The blend containing EOC with52 wt.% of octene (with the finest morphology) crystallizedthe fastest, while the blend containing EOC with 28 wt.% ofoctene (with the largest particles) crystallized the slowest.

Fig. 7 shows the effect of molecular weight of EOC oncrystallization (for constant octene content 37 wt.% in EOC).Both curves have a parabolic shape with a minimum at180k g mol�1. The fastest of these three blends was the onecontaining EOC with 229k g mol�1 having PP as a contin-uous phase with dispersed EOC particles. Only a littleslower was the blend containing EOC with 115k g mol�1

which has continuous phase EOC with dispersed PP parti-cles. The slowest of these three blends was the one con-taining EOC with Mn being 180k g mol�1. The uniquestriped structure created the largest obstacles to growingPP lamellae, resulting in the slowest crystallization. Theinteresting question is: ‘‘Can a blend with EOC matrix orstriped structure form spherulites (such as in the case ofEOC-28-131 and EOC-37-180?’’

The answer is ‘‘Yes’’ and is shown in Fig. 8a and b.However, the spherulites have rather disturbed, imperfectstructure. PP lamellae always find a bridge or way aroundthe obstacle so that crystallization can continue. The largerthe obstacle the longer time it takes to go around it and theslower crystallization is observed.

This is schematically shown in Fig. 9. If the straight linein Fig. 9a measured 10 mm, then the line through a structurewith smaller particles (Fig. 9b) would measure 12.8 mm and

P. Svoboda et al. / Polymer Testing 28 (2009) 215–222 221

a line through a structure with large particles (Fig. 9c)would measure 16.7 mm. Larger particles, according to thismodel, would slow down the crystallization kinetics about30%. When we compare the kinetics for the fastest EOC-52-130 and the slowest EOC-37-180 (listed in Table 2) we get2.763/1.884 ¼ 1.47, which is about 47% increase in crystal-lization kinetics. The change of crystallization kineticsaccording to the model and measured numbers are in goodcorrelation.

The influence of both octene content and molecularweight on crystallization kinetics is summarized in 3Dgraphs shown in Fig. 10a and b. Both methods, opticalmicroscopy and DSC, are in agreement.

Isotactic PP has a relatively high melting point (about165 �C) that allows rather high service temperatures, andEOC gives the blend softness (rubber-like feeling) andincreases impact strength. The only disadvantage of EOC isa low melting point (high octene content 40–50 �C, lowoctene content around 90 �C) that could limit the servicetemperature of the blend. This shortcoming can be

Fig. 11. Spherulites after 4 min at Tc ¼ 135 �C in 50/50 blend of PP/ EOC-37-180 (a) and EOC-52-130 (b) by polarized optical microscopy (1 min pre-heating at 200 �C).

1.8

2.0

2.2

2.4

2.6

2.8

30

35

4045

50

120140

160180

200220

G /

µµm m

in

-1

wt%

of octe

ne

a

104

106

108

110

120140

160180

200220

3035

4045

50

Tc / °C

M

n x 1

0-3

/ g m

ol

-1

wt% of octene

b

Mn x 10-3

/ g mol-1

Fig. 10. Crystallization kinetics from optical microscopy (a) and from DSC (b)as a function of octene content and molecular weight Mn of EOC for PP/EOC50/50 blends.

overcome by silane grafting followed by water cross-link-ing [30,31]. Such thermoplastic vulcanizates have greatpotential in the automotive industry [27]. Cross-linkingleads to better thermal resistance and can be done also byan electron beam, however, we have achieved better tensilestrength and elongation (at 25 �C and also at 150 �C) for thesamples cross-linked by silane, as described in our patentapplication [32] concerning PP/EOC foam products.

4. Conclusion

TEM study has proved that the morphology of the PP/EOC blends depends greatly on both octene content inthe EOC and also on molecular weight of the EOC. Withincreasing octene content the particles in the blend aregetting smaller. This observation is in agreement withCarriere et al. [21] who have shown that increasingoctene content in EOC has lowered the interfacial tensionin PP-EOC systems. When the Mn of EOC is low, the EOCbecomes the matrix (with PP dispersed particles). Whenthe Mn of EOC is high, PP is the continuous phase andEOC forms the particles.

Optical microscopy together with DSC measurementhas revealed that crystallization kinetics was fastest for theblend with EOC-52-130 that had the smallest particles andPP as continuous phase. The relation of crystallizationkinetics with particle size was explained by the model. The

P. Svoboda et al. / Polymer Testing 28 (2009) 215–222222

larger the particles, the longer is the path for growingcrystal lamellae and, therefore, slower crystallizationkinetics. The blend containing EOC-52-130 with very finemorphology (by TEM) also had many more spherulites(compare optical microscopy pictures of EOC-52-130 andEOC-37-180 in Fig. 11 after 4 min). This fact was confirmedby DSC exhibiting the crystallization temperature Tc ataround 110 �C, much higher than for other blends. Fastercrystallization with many small spherulites has two posi-tive effects. First, the injection molding cycle can be shorterwhich can save money during production. Second, smallersize of the spherulites increases the impact strength, as wasshown for pure PP [33].

Acknowledgment

This work has been supported by the Ministry ofEducation of the Czech Republic as a part of the project No.VZ MSM 7088352102.

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